Optical data transmission using light wave guides (optical fibers) has become increasingly important because of high transmission capacity, low attenuation rates and decreasing fiber costs. For producing a complete transmission system, it is necessary to provide pluggable couplers, so-called optical plugs, which when used will not significantly degrade the optical quality of the transmission line and will not introduce significant attenuation.
At the very small geometrical dimensions of the light wave guides, ranging from several .mu. (in the case of so-called monomode fibers) to a maximum of 100.mu. (in the case of multi-mode fibers), the coupling of two fibers has required extreme mechanical accuracy in the plug dimensions. Proposals known from the art for making optical plugs (see, for example, the article by M. Kaiser in Elektronik 1979, Vol. 8, page 90) are based on the principle of mutual alignment of the fiber ends, using highly accurate and thus expensive precision means.
A second approach proposed for solving this problem consists in providing an optical imaging system between the two fibers to be connected. Examples of such a system may be seen from Deutsche Offenlegungsschriften 29 05 360 and 25 46 861. Because of the high accuracy requirements, such imaging systems are also elaborate, expensive and difficult to align.
Finally, for by-passing the mutual alignment of very small dimensions, it has been proposed to provide the fibers to be connected with thickened end pieces, thus obtaining larger stop faces which are less difficult to align. An example of such optical plugs is specified in U.S. Pat. Ser. No. 3,944,327. To permit light propagation in the thickened end pieces to proceed unhampered, the refractive index profile in these end pieces must be of a particular (e.g., parabolic) shape. Although this is possible in principle, using, for example, ion implantation, the manufacturing expenditure is considerable.